This invention relates to a corona discharge apparatus (corona apparatus) and a method for the corona treatment. More particularly, it relates to a corona apparatus and a method for the corona treatment which are suitable for use in hydrophilizing (i.e. imparting hydrophilic properties to) a hydrophobic surface in the case of providing a hydrophilic layer such as a photographic emulsion layer on a support having the hydrophobic surface.
Photosensitive materials are manufactured generally by providing at least one coating layer of a silver halide emulsion on a support. As the supports, there are used, in accordance with the purpose, various materials having a hydrophobic surface, such as polyethylene-coated paper, polyethylene terephthalate film and the like. Because of its hydrophilic nature, a photographic layer does not strongly adhere to a hydrophobic surface, if coated directly thereon. It is a general practice, therefore, to hydrophilize the surface of a hydrophobic support before coating. The hydrophilization is effected by various means including the provision of a subbing layer by coating the support surface with a composition which adheres to both the hydrophobic surface and the hydrophilic coating layer, and the surface treatment such as corona treatment, flame treatment, exposure to ultraviolet rays, or glow-discharge treatment. Among these, in view of the ease of operation, safety and cost, the corona treatment has recently been in wide use, particularly for polyethylene-coated paper.
The corona treatment, however, offers several difficulties. One of the important difficulties associated with the manufacture of photosensitive materials is the occurrence of non-uniformity in photographic coating layer resulting from uneven distribution of the electrical charge which, in turn, is originated in non-uniform discharge. Since the photographic layer is a thin layer of several to several tens microns in thickness, the non-uniform coating severely affects the photographic characteristics, thus deteriorating markedly the photographic products in quality.
The means heretofore proposed for the elimination or diminution of the coating non-uniformity include improvement of the coating composition so as to make it less sensitive to non-uniform corona electrical charging; storage of the corona-treated material for a certain period of time before coating so as to make uniform the distribution of charge; and a D.C.-treatment of the corona-treated material before coating by means of a strong D.C. field so as to make uniform the distribution of charge. The improvement of the coating composition not only accompanied other restrictive conditions but also failed in sufficiently eliminating the coating non-uniformity. The temporary storage before coating of the corona-treated material involved an increase in both the quantity of intermediate products and the number of operation steps, resulting in an increase in cost. The D.C.-treatment revealed a disadvantage of insufficient flattening of the charge distribution.